1. Field of the Invention
This invention relates to new lubricating compositions embodying a new mechanism of lubrication; and, more particularly, to lubricating compositions for slow speed and highly loaded bearings, such as for the highly loaded journal bearings in bits used to drill earth formations.
2. Description of the Prior Art
Lubricating a drill bit drilling in subterranean earth formations is one of the most severe and demanding set of conditions ever posed for a lubricant. The drilling takes place in an abrasive environment of mud and rock particles deep below the earth's surface. The bit is subjected to generally slow speeds and extremely high loads, sufficient to overcome the compressive strength of the rock formations in order to drill the formations. The bit most often experiences great fluctuations in load and speed as it vibrates and bounces while drilling the formation. The bearings in the bit thus are subjected to very high unit loads resulting from the loads necessary to crush the rock formation and extremely high shock loads resulting from vibration and bouncing of the drill bit. All of the horsepower delivered to the bit cutting surfaces must be delivered through the bit bearings. Thus, it can be seen that the lubricant used to lubricate the bit bearings must be capable of minimizing or preventing scoring, galling, and wear of the bearing surfaces subjected to such extreme operating conditions.
Rock bits have generally been lubricated with greases. Greases, commonly, are made by thickening an oil, generally, with a metal salt of a fatty acid, known in the art as a soap. The finished grease may also include extreme pressure additives such as lead dithiocarbamate or organic lead-sulfur compounds, anti-wear additives such as zinc dithiophosphate or antimony phosphorodithioate, anti-oxidation additives such as diphenylamine or phenothiazine, tackiness agents such as polybutene polymers or acrylate polymers, viscosity improvers such as isobytylene polymers or acrylate copolymers, and dyes. In addition, the grease may contain solid materials or fillers such as graphite, molybdenum disulfide, metal oxides, or powdered metals for improved load carrying ability by forming a solid layer interposed between the bearing surfaces.
Surface speeds in rock bit bearings are generally so slow that fluid film or hydrodynamic lubrication cannot be continuously maintained, resulting in the bearings operating in a starved-film or boundary mode of lubrication.
Lubricants using commonly available extreme pressure and anti-wear additives have yielded only slight improvements in bit bearing performance over the greases used in rock bits. That the conventional extreme pressure agents have yielded only incremental improvements in bearing performance has been attributed to the extremely severe combination of slow speeds and high unit loading commonly encountered in rock bit bearings and a very small supply of lubricant.
Subsequently, lubricants were developed utilizing increased quantities of solids or fillers in the greases. The solid was generally molybdenum disulfide. The function of the filler was to maintain a physical separation of the bearing surfaces by a low shear material. For hydrodynamic or elasto-hydrodynamic lubrication, most lubricants contain 2-5% solids, such as molybdenum disulfide. However, for successful rock bit lubrication, grease lubricants contain 10-60% solids, mainly molybdenum disulfide.
One such lubricant for a journal bearing is that described in U.S. Pat. No. 3,935,114 and generally comprises a calcium acetate complex grease with molybdenum disulfide and metallic oxides. Another such lubricant that was previously used for journal bearing rock bits included 85% of a lithium-12 hydroxystearate chassis grease and 5% by weight each of silver powder, copper powder, and graphite.